GVB–RP: A reliable MCSCF wave function for large systems

We have developed a version of generalized valence bond (GVB) that overcomes the major weakness of the perfect pairing approximation without requiring a full transformation of the integrals at each step of the self‐consistent orbital optimization. The method, called generalized valence bond–restricted pairing (GVB–RP), describes properly the dissociation of up to triple bonds and provides smooth potential energy surfaces for most chemical reactions. The wave functions obtained are a good starting point for more sophisticated computational techniques. The applicability of the method is illustrated with a few simple examples including multiple‐bond dissociations, transition states for symmetry allowed, symmetry forbidden, and radical reactions, as well as reactions at a transition‐metal center. The cost of the method compares well with other self‐consistent correlated techniques. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 1–22, 1999     

Comparison of basis set superposition error corrected perturbation theories for calculating intermolecular interaction energies

Recently, two different but conceptually similar basis set superposition error (BSSE) free second‐order perturbation theoretical schemes were developed by us that are being based on the chemical Hamiltonian approach (CHA). Using these CHA‐MP2 and CHA‐PT2 methods, a comparison is made between the a posteriori and a priori BSSE correction schemes at the correlated level. Sample calculations are presented for four hydrogen bonded complexes (HFH₃N, HFH₂O, H₂SHF, and H₂OHCl) in nine different basis sets (from 6–31G to TZV**++). The results show that the BSSE content is very significant in the interaction energy if electron correlation is accounted for, so removing the BSSE is very important. The differences of the two perturbational theories discussed are connected solely with the different one electron orbital sets used for building up the unperturbed single determinant wave function. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 274–283, 1999     

Linear expansions of correlated functions: Variational Monte Carlo case study

The relative performance of trial wave functions expressed as linear combination of correlated exponentials has been tested on a variety of systems. The results are compared against other correlated functions commonly used in the literature to assess the capabilities of the proposed ansatz. A possible departure from the simple exponential functional form used in previous works is discussed, along with its advantages and drawbacks. We also discuss how to implement an efficient optimization procedure for this correlated basis set. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 74: 23–33, 1999     

Analytical Hartree–Fock wave functions subject to cusp and asymptotic constraints: He to Xe,Li+ to Cs+, H− to I−

Analytical, variational approximations to Hartree–Fock wave functions are constructed for the ground states of all the neutral atoms from He to Xe, the cations from Li⁺ to Cs⁺, and the stable anions from H⁻ to I⁻. The wave functions are constrained so that each atomic orbital agrees well with the electron–nuclear cusp condition and has good long‐range behavior. Painstaking optimization of the exponents and principal quantum numbers of the Slater‐type basis functions allows us to reach this goal while obtaining total energies that, at worst, are a few microHartrees above the numerical Hartree–Fock limit values. The wave functions are freely available by anonymous ftp from okapi.chem.unb.ca or upon request to the authors. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 491–497, 1999     

Analytic correlated wave functions in momentum space and related properties of helium like ions

We obtain analytic correlated wave functions in momentum space as the Fourier transform of correlated wave functions which are able to incorporate almost all of the correlation energy for the ground-state of two-electron atoms. Then we study the atomic momentum-density, the Compton profile and the elastic and inelastic scattering factors for this kind of wave functions. The scattering factors are also compared with the ones provided by a more accurate correlated wave function. All the calculations can be analytically performed, provided the correlated wave function in position space has been determined.     

Degradation of polymer coating systems studied by positron annihilation spectroscopy. II. Correlation with free radical formation

The photo‐degradation of polymer coating systems due to irradiation by UV and Xenon light sources is studied using positron annihilation spectroscopy and electron spin resonance (ESR). Doppler broadened spectra of positron annihilation, as a function of slow positron implantation energy and ESR spectra, are measured in two types of polyurethane which were exposed, ex situ, to UV irradiation for up to 800 h. The UV irradiation systematically decreases the S parameter as a function of exposure duration and increases the ESR signals. Thus, significant S parameter decrease is correlated with the ESR signal increase resulting from photo‐degradation of polymers due to UV irradiation. Parallel in situ positron annihilation and ESR experiments are performed as a function of Xenon light exposure for up to 100 min. These results show that the photo‐degradation of the polyurethane coatings involves initial free‐radical formation, which is correlated with the subnanometer defects detected by positron annihilation spectroscopy. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1289–1305, 1999     

IR laser‐induced decomposition of disiloxane for chemical vapour deposition of poly(hydridosiloxane) films

Continuous‐wave CO₂‐laser‐induced gas‐phase decomposition of H₃SiOSiH₃, dominated by elimination and polymerization of transient silanone H₂SiO and yielding silane and hydrogen as side‐products, represents a convenient process for chemical vapour deposition of poly(hydridosiloxane) films. Copyright © 1999 John Wiley & Sons, Ltd.     

Description of correlated densities for few‐electron atoms by simple functional forms

Simple analytical functional forms for the electron density of two‐ and three‐electron atoms which reproduce fairly the correlated (exact) values are presented. The procedure is based on the fitting of an auxiliary f(r) function which has adequate properties for this purpose and can be extended to more complex atoms. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 443–454, 1999     

Structure of molecular energy levels of homonuclear diatomic molecules

A new approach is given for the systematic prediction of the low‐lying electronic states of homonuclear diatomic molecules. The approach is based on the bond order and the energy levels of the separated atoms. The asymptotic wave functions are derived from two atomic wave functions by using new operators defined as linear combinations of certain ladder operators. We show that the low angular moment states tend to have a high bond order in the states derived from an asymptote. The observed low‐lying states of C₂, C, Sc₂, and Ti₂ molecules agree with the predictions. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 597–604, 1999     

A quantum mechanical approach to electrochemical behavior of spirochromics

Fully optimized semiempirical quantum‐chemical calculations of photochromic spiropyrans are presented. The vertical ionization potentials are calculated and their variation with substitutions are correlated to experimental oxidation potentials. The effects of the substitutions are studied and the partial charges on indoline and pyran components generated by HOMO are found to be responsible for the variations. The deactivating groups on the indoline ring system and deactivating groups on the pyran system increase the ionization potential and, consecutively, the oxidation potential. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 111–117, 1999     

Electrochemical Determination of Some Organic Pollutants Using Wall‐Jet Electrode

Determination of organic pollutants in trace level in a flow stream was developed using a wall‐jet electrode system and square‐wave stripping principles. Pesticides such as endosulfan, isoproturon and carbendazim, o‐chlorophenol and a benzidine bisazo dye, direct orange 8 were chosen for this study. The optimum conditions for adsorptive square‐wave stripping voltammetry were arrived at and the peak current responses of analyte were correlated over flow rate. Calibration plots were obtained by correlating the peak currents with concentration under optimum conditions. The best determination range, 1×10^?4 to 4.0 mg/mL for endosulfan and 1×10^?4 to 1.5 mg/mL for the remaining pollutants in a flowing stream was obtained with adsorptive square‐wave stripping technique.     

Theoretical studies of force fields and IR spectra of isocytosine

The optimized geometries, complete harmonic force fields, and infrared intensities of isocytosine tautomers, amino‐hydroxy and amino‐oxoN(1)H, were calculated at the ab initio Hartree–Fock level using the 6‐31G* basis set. The theoretical force fields were scaled by empirical scale factors, which were determined by fitting to the IR spectrum of the amino‐oxo form and were then transferred to the amino‐hydroxy form. The average deviations between experimental and computed frequencies are 7.6 cm⁻¹ for amino‐oxo and 9.5 cm⁻¹ for amino‐hydroxy, respectively. The assignments of the fundamental frequencies and the transferability of the force constant scale factors are also presented. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 53–60, 1999     

Vibrational spectrum of Li3 first‐excited electronic doublet state: Geometric‐phase effects and statistical analysis

We present J=0 calculations of all bound and pseudobound vibrational states of Li₃ in its first‐excited electronic doublet state by using a realistic double many‐body expansion potential‐energy surface and a minimum‐residual filter diagonalization technique. The action of the system Hamiltonian on the wave function was evaluated by the spectral transform method in hyperspherical coordinates. Calculations of the vibrational spectra were carried out both without consideration and with consideration of geometric‐phase effects. Dynamic Jahn–Teller and geometric‐phase effects are found to play a significant role, while the calculated fundamental symmetric stretching frequency is larger by 8.3% than its reported experimental value of 326 cm⁻¹. From the neighbor‐spacing distributions of the levels, it is observed that the title vibrational spectrum is quasiregular in the short range and quasi‐irregular in the long range. By the Δ₂ standard defined in this article, it is found that the spectra are more nonuniform than those of the “trough” states for the ground electronic state. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 89–109, 1999     

Gas transport properties of soluble poly(amide imide)s

The influence of the molecular structure of five soluble poly(amide imide)s (PAI)s on their gas transport properties for carbon dioxide, oxygen, nitrogen, and methane has been studied. Permeabilities, diffusivities, and solubilities were determined by time lag measurements and correlated to chain packing and mobility as well as to polymer gas interaction. The PAIs were characterized by small‐ and wide‐angle X‐ray scattering. Molar masses and polymerization degrees were measured by light scattering. Additionally, glass transition temperatures, densities, and persistence lengths were determined. Pressure‐ and temperature‐dependent gas transport measurements have been done. It was found that the permeability is increasing with the diffusion coefficient which can be related to the fractional free volume. PAIs containing cardo diamines show higher diffusivities and permeabilities than poly(amide imide)s containing linear aromatic diamines due to higher fractional free volumes. The solubilities for PAIs containing the same imide compound correlate with the molar cohesive energy density. The exchange of hydrogen to fluorine atoms at one aromatic ring of the diamine increases the fractional free volume and cohesive energy density and, in consequence, the diffusion and solubility coefficient. Arrhenius behavior was observed for temperature dependence and decreasing permeability with increasing pressure. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2183–2193, 1999     

An open‐source framework for analyzing N‐electron dynamics. II. Hybrid density functional theory/configuration interaction methodology

In this contribution, we extend our framework for analyzing and visualizing correlated many‐electron dynamics to non‐variational, highly scalable electronic structure method. Specifically, an explicitly time‐dependent electronic wave packet is written as a linear combination of N‐electron wave functions at the configuration interaction singles (CIS) level, which are obtained from a reference time‐dependent density functional theory (TDDFT) calculation. The procedure is implemented in the open‐source Python program det CI@ORBKIT, which extends the capabilities of our recently published post‐processing toolbox (Hermann et al., J. Comput. Chem. 2016, 37, 1511). From the output of standard quantum chemistry packages using atom‐centered Gaussian‐type basis functions, the framework exploits the multideterminental structure of the hybrid TDDFT/CIS wave packet to compute fundamental one‐electron quantities such as difference electronic densities, transient electronic flux densities, and transition dipole moments. The hybrid scheme is benchmarked against wave function data for the laser‐driven state selective excitation in LiH. It is shown that all features of the electron dynamics are in good quantitative agreement with the higher‐level method provided a judicious choice of functional is made. Broadband excitation of a medium‐sized organic chromophore further demonstrates the scalability of the method. In addition, the time‐dependent flux densities unravel the mechanistic details of the simulated charge migration process at a glance. © 2017 Wiley Periodicals, Inc.     

Synthesis and quantitative structure–activity relationship of a new series of chiral 4‐alkoxycarbonyl‐2‐(alkylamino)‐1,3,2‐oxa or thiazaphospholidine‐2‐ones

Twenty‐one of the chiral 4‐alkoxycarbonyl‐2‐(α‐alkyl‐α‐ethoxycarbonyl methylamino)‐1,3‐2‐thia or oxazaphospholidine‐2‐ones have been synthesized by cyclization of L‐serine or L‐cysteine ethyl or n‐octyl ester with phosphoryl chloride followed by reaction with a suitable L‐amino acid ethyl ester. Proton NMR, IR, and mass spectra of these compounds have been discussed in detail. These compounds inhibited up to 68.52% of acetylcholinesterase (AChE) at the 1 ppm concentration level. Regression analysis showed that AChE inhibition was determined by both the steric and electronic effects of the alkyl groups of the amino acid. The enzyme inhibition correlated directly with the steric bulk of the alkyl groups, indicating a steric requirement for maximizing inhibitor–enzyme interaction and an inverse relationship with the electron‐donating ability of the alkyl groups. This supports the concept of a nucleophilic attack mechanism of a hydroxyl group of a serine amino acid in the enzyme active center on the partially positive phosphorus atom of the oxazaphospholidines and thiazaphospholidines, with correlation coefficients of 0.999 and 0.838, respectively. Results also indicated that the steric requirement was more important than the electronic factor in affecting the inhibition process, which explained the high activity of compounds containing the isoleucine moiety. The high AChE inhibition activity of these compounds and the expected nontoxic products of their in vivo hydrolysis make them eligible for pesticidal application. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10: 475–480, 1999     

Use of SAXS and linear correlation functions for the determination of the crystallinity and morphology of semi‐crystalline polymers. Application to linear polyethylene

The use of correlation functions to obtain the morphological parameters of crystalline‐amorphous two‐phase lamellar systems is critically reviewed and extended. It is shown that processing of the experimental SAXS‐patterns only significantly affects the curvature of the autocorrelation triangle and that the parameters of the corresponding ideal two‐phase structure can be determined independently of the data processing procedure. The methods to be used depend on the normalization of the correlation function. The validity of the formulation is illustrated for a sample of linear polyethylene, cooled and heated at 10°C per min. Crystallite thickening during crystallization and surface melting during heating are observed. The overall crystallinity and the fraction of semi‐crystalline stacks during crystallization and melting are determined quantitatively as a function of temperature using the total scattering power of the corresponding ideal two‐phase structure, correlation functions, and a scaling procedure. Absolute intensities are not required. The SAXS results are confirmed by independent techniques (DSC, WAXD, and SALLS). During crystallization, amorphous regions are present outside the semi‐crystalline regions because growing spherulites do not fill space completely. During melting, larger amorphous regions develop in the spherulites because of the complete melting of stacks. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1715–1738, 1999     

Perturbational approach to the electron correlation cusp applied to helium‐like atoms

A recently proposed perturbational approach to the electron correlation cusp problem 1 is tested in the context of three spherically symmetrical two‐electron systems: helium atom, hydride anion, and a solvable model system. The interelectronic interaction is partitioned into long‐ and short‐range components. The long‐range interaction, lacking the singularities responsible for the electron correlation cusp, is included in the reference Hamiltonian. Accelerated convergence of orbital‐based methods for this smooth reference Hamiltonian is shown by a detailed partial wave analysis. Contracted orbital basis sets constructed from atomic natural orbitals are shown to be significantly better for the new Hamiltonian than standard basis sets of the same size. The short‐range component becomes the perturbation. The low‐order perturbation equations are solved variationally using basis sets of correlated Gaussian geminals. Variational energies and low‐order perturbation wave functions for the model system are shown to be in excellent agreement with highly accurate numerical solutions for that system. Approximations of the reference wave functions, described by fewer basis functions, are tested for use in the perturbation equations and shown to provide significant computational advantages with tolerable loss of accuracy. Lower bounds for the radius of convergence of the resulting perturbation expansions are estimated. The proposed method is capable of achieving sub‐μHartree accuracy for all systems considered here. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Small‐angle X‐ray scattering from poly(methylmethacrylate) in aqueous solutions of t‐butyl alcohol

Segment‐segment interaction of poly(methylmethacrylate) in t‐butyl alcohol‐water mixtures in poor solvent regime was studied. From the small‐angle X‐ray scattering measurements of semidilute solution range, the binary and ternary cluster integrals of polymer segments were determined from concentration dependence of the correlation length at various temperatures just above the upper critical solution temperature. We have calculated the contributions of the segment–segment interaction to the entropy and enthalpy from the measured temperature dependence of these interaction parameters and found that both quantities are negative and decrease with decreasing t‐butyl alcohol content. FT‐IR absorption peak of carbonyl group of poly(methylmethacrylate) shifts to the lower frequency with increasing water content. The implications of these findings are discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2195–2199, 1999